In the prior art, addition curing type silicone rubber compositions are typically molded by press molding, transfer molding, and injection molding techniques. Under the current demand for cost reduction and mass production, it is desired to increase the cure rate of rubber to shorten the molding cycle. In the case of addition curing, however, the cure rate is increased at the sacrifice of a pot life. A silicone rubber composition is prepared by admixing its components. No problems arise if the silicone rubber composition is molded immediately after admixing. However, if the molding machine is stopped and kept quiescent for a long period of time, the composition will thicken during that period. When the molding machine is operated again, clogging or gelation can occur due to thickening or a short pot life before the composition reaches the mold, causing detrimental effects on molded parts.
To impart physical properties such as strength and durability to silicone rubber, the blending of reinforcing silica is requisite. In the application where transparent parts are necessary, large amounts of reinforcing silica must sometimes be blended in order to provide a high hardness. After blending, the silica starts to agglomerate with the lapse of time, causing thickening. In the case of an addition curing type silicone rubber composition comprising a base oil, a crosslinking component (organohydrogenpolysiloxane) and an addition reaction catalyst, the silica blended gives rise to the problem of a short pot life. There is a desire to overcome these problems.